Lasers

ABSTRACT

A resonant power circuit which supplies pulsed power to a load, such as a pulsed laser, from a d.c. supply by cyclically charging a capacitor. The capacitor is discharged through the load on closure of a switch which should then open to allow the capacitor to re-charge. Occasionally the switch fails to open and the circuit of the invention then develops and applies a reverse voltage to the switch to force it to become an open circuit.

BACKGROUND OF THE INVENTION

This invention relates to an electric circuit which provides protectionto a power source supplying power to a load through a resonant circuit.In such ciruits a cyclical operation takes place in which a directcurrent power supply charges a capacitor and the power stored in thecapacitor is then discharged through the load. The discharge suppliesoperating power to the load and the present invention applies inparticular to a load which needs repetitive discharges over an extendedperiod or for continuous operation.

In such circuits the discharge of the capacitor through the load can beinitiated by a switch device which connects the fully-charged capacitoracross the load. At the appropriate time it is necessary for the switchdevice to become open circuit so that the capacitor can recharge and theregular operation of the circuit requires that this should occur.

If the switch device fails to become open circuit it effectivelypresents a short circuit to the power supply and unless the switchbecomes open circuit within a predetermined time, the protectivecircuits of the power supply will cause the power supply to switch offto prevent it being damaged. The present invention provides a protectivecircuit to prevent just such an occurrence.

SUMMARY OF THE INVENTION

According to the invention there is provided an electrical circuitcomprising first capacitive means arranged to be charged from a powersupply, switch means arranged to close for discharging said firstcapacitive means through a load and then to open to enable said firstcapacitive means to re-charge, characterised in that second capacitivemeans are connected across said switch means and current limiting meansare arranged in the circuit between the power supply and said secondcapacitive means, whereby upon continued closure of said switch meanssaid second capacitive means first discharges through said switch meansand then applies a reverse voltage to said switch means causing openingof said switch means.

Preferably, said power supply is prevented from charging said secondcapacitive means in the event of continued closure of said switch meansby said current limiting means.

Preferably also, means are provided for discharge of the energy in saidcurrent limiting means.

Preferably also, a detector is arranged to indicate if operation of saidcircuit occurs in excess of a predetermined rate.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:-

FIG. 1 is a circuit diagram for a laser discharge resonant circuitconstructed in accordance with the invention, and

FIGS. 2 to 4 are voltage and current wave form diagrams.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to the circuit outside (to the left and right of) thetwo broken lines of FIG. 1, there is shown a capacitor C1 arranged to becharged from a direct current voltage supply V0. When voltage V0 isapplied current flows through inductor L1 and diode D1 to chargecapacitor C1. The voltage V1 (FIG. 2) across capacitor C1 rises towardsV0, the current I1 (FIG. 3) reaches a maximum and starts to decay.Inductor L1, which initially resists the current I1 flowing when voltageV0 is first applied, now resists decay of this current with the resultthat the voltage across capacitor C1 continues to rise and eventuallyreaches twice V0.

At time t1 thyratron T closes and connects capacitor C1 directly acrossthe load. The load in this case is a pulsed metal vapour lase using amaterial such as copper as the vapour lasing medium. The charge whichhas been developed across capacitor C1 causes a discharge to travel downthe discharge tube of the laser initiating lasing. Normally thyratron Tthen becomes open circuit after a short period and capacitor C1 is thenrecharged from the d.c. power supply. This mode of operation continuesat a pulsed repetition rate of about 20 KHz.

Capacitor C2 is connected across the power supply for smoothing purposesand when thyratron T closes, capacitor C2 does not discharge through itto any great extent because inductor L1 limits the speed at which thecurrent from capacitor C2 can build up through the short circuit ofthyratron T.

From time to time quite randomly, thyratron T malfunctions and remainsshort circuited when the system requires it to go open circuit. In thesecircumstances the discharge from capacitor C2 continues and the shortcircuit which is continued to be applied across the power supply causesprotective devices (not shown) of the power supply to operate andisolate the circuit thus causing the laser to cease operation.

Metal vapour lasers which are caused to stop operating then have to befully re-initiated and this can take up to an hour to complete beforethe laser can be made to operate again. Unwanted closedown for thisreason can be a major disadvantage with this kind of laser.

The components within the two broken lines in FIG. 1 constitute acircuit which provides protection to the system in the case of thyratronmalfunction of this kind.

When the cycle starts capacitors C1, C2 and C3 are all charged from thevoltage V0. When thyratron T closes capacitor C1 is discharged throughthe load but capacitors C2 and C3 are prevented from discharging to anygreat extent by the inductances L1 and L2.

If thyratron T malfunctions and remains closed after the system requiresit to go open circuit, capacitor C3 discharges through thyratron T whilecapacitor C2 is still prevented from discharging to any great extent dueto the higher inductance of L2 as compared to L1. When the voltage V3(FIG. 4, note the different timescale) across capacitor C3 reaches zero(at approximately time 10 * t1) and if thyratron T is still closed, thecurrent I3 flowing starts to decay but this decay is resisted by theinductor L1; as a result, the voltage V3 reverses and the current I3decays to zero and tries to reverse. The reverse voltage V3 is appliedacross thyratron T forcing it to become open circuit.

Because current flow from capacitor C2 is prevented from building up byinductor L2, capacitor C2 is discharged only slowly and the current fromit is insufficient to affect the over-swing of voltage across capacitorC3 described above.

Operation of the circuit as described above does, however, build upenergy in inductor L2 and after thyratron T has again become opencircuit, this energy is discharged into resistors R1 and R2 throughdiode D2. Also, light emitting diode D3 flashes to give a visibleindication of the discharge of L2 and detector D4 is arranged to countthe rate of malfunction of the circuit. If this exceeds a predeterminedrate, of the order of eight times per day, detector D4 gives anindication that the laser ought to be closed down for maintenance.

Recharging of capacitors C1 and C3 will take place in the normal mannerafter operation of the circuit to force open circuit of the thyratron T.

One additional aspect is that capacitor C3 would be capable ofrecharging to more than voltage V0 (up to twice voltage V0) by currentflow from capacitor C2 through inductor L2; this would interfere withthe desired functioning of the thyratron T. However, for low/averagecurrent circuits resistor R1 provides sufficient damping to limit thiscurrent and limit the voltage to V0.

In high power circuits diode D2 and resistor R2 remove any excessvoltage from capacitor C3 and, in this case, resistor R1 can be reducedto zero or at least to the resistance provided by the windings ofinductor L2. Resistor R2 is then chosen in value sufficient to damp outquickly the current which would circulate through inductor L2, diode D2and resistor R2.

Typical circuit values of these components are:

    ______________________________________                                        L1                100 mH                                                      L2                1.5 H                                                       C1                8 nano Farads                                               C2                10 micro Farads                                             C3                2 micro Farads                                              R1                0- 10 K Ohms                                                R2                0- 10 K Ohms                                                ______________________________________                                    

We claim:
 1. An electrical circuit comprising first capacitive meansarranged to be charged from a power supply, a thyratron arranged toclose for discharging said first capacitive means through a load andthen to open to enable said first capacitive means to recharge, whereinthe improvement comprises second capacitive means connected across saidthyratron and current limiting means arranged in the circuit between thepower supply and said second capacitive means, whereby upon continuedclosure of said thyratron said second capacitive means first dischargesthrough said thyratron and then applies a reverse voltage to saidthyratron causing opening of said thyratron.
 2. An electrical circuit asclaimed in claim 1, wherein said power supply is prevented from chargingsaid second capacitive means in the event of continued closure of saidthyratron by said current limiting means.
 3. An electrical circuit asclaimed in claim 2, wherein means are provided for discharge of theenergy in said current limiting means.
 4. An electrical circuit asclaimed in claim 1, wherein a detector is arranged to give a visibleindication when said thyratron is caused to go open circuit.
 5. Anelectrical circuit as claimed in claim 1, wherein a detector is arrangedto indicate if continued closure of said thyratron occurs in excess of apredetermined rate.
 6. In combination with a first capacitor arranged tobe charged from a power supply, a load connected in series with saidfirst capacitor and a thyratron connected in parallel with saidseries-connected first capacitor and said load, whereby said thyratronunder normal operating conditions periodically closes to discharge saidfirst capacitor through said load and opens after a predeterminedinterval to terminate said discharge, said thyratron malfunctioning fromtime to time in a random manner so as to remain closed after saidpredetermined interval has elapsed thereby permitting continueddischarge of said first capacitor through said load; an electricalcircuit for forcing said thyratron to open in the event of saidmalfunction comprisinga first inductor having one end connected to thejunction of said first capacitor and said thyratron; a second inductorconnected between the other end of said first inductor and said powersupply, the inductance of said second inductor being greater than theinductance of said first inductor; and a second capacitor connectedbetween the junction of said first and second inductors and the junctionof said thyratron and said load, said second capacitor being chargedfrom said power supply through said second inductor and dischargedthrough said first inductor and said thyratron when said thyratronmalfunctions by remaining closed after said predetermined interval haselapsed, the polarity of the voltage across said second capacitorreversing after said predetermined interval has elapsed thereby forcingsaid thyratron to open and terminate the discharge of said firstcapacitor through said load.
 7. An electrical circuit as claimed inclaim 6 wherein said load is a metal vapor laser.
 8. An electricalcircuit as claimed in claim 6 which further comprises a resistor coupledacross said second inductor to discharge the energy stored thereinduring charging of said first and second capacitors.
 9. An electricalcircuit as claimed in claim 8 which further comprises detecting meanscoupled across said resistor, said detecting means detecting dischargeof the energy stored in said second inductor.
 10. An electrical circuitas claimed in claim 9 wherein said detecting means is a light emittingdiode.
 11. An electrical circuit as claimed in claim 9 wherein a furtherdetecting means is coupled to said detecting means, said furtherdetecting means indicating when the discharge of energy from said secondinductor exceeds a predetermined rate.